Radiance House Publishing released Secrets of Pact Magic and Villains of Pact Magic a couple of years ago as supplements for 3.5 Dungeons & Dragons. Dario Nardi, the man behind the books, recently updated the series with a free 4 page Pathfinder conversion guide. The guide makes the rules supplements more accessible for groups using Paizo’s successful iteration of D&D 3.5.
The Pathfinder Chronicles Campaign Setting is probably one of the best setting books that I have seen in the past few years. The official Pathfinder RPG campaign setting is the world of Golarion, where all the Pathfinder Adventure Paths and Pathfinder Society modules take place. Golarion and its cosmology truly offer something for everyone.